Certain water-soluble allyl containing monomers, such as N-2-(allylcarbamato) aminoethyl imidazolidinone, generally known as wet adhesion monomers, dramatically improve the adhesion of latex polymers to various substances under conditions of high humidity. These monomers impart improved moisture resistance to acrylate copolymers prepared with them. However, they have a tendency to undergo chain transfer and drastically reduce latex polymer chain length (this is shown by, among other things, the high chain transfer constant of the allyl monomers and the low reduced specific viscosity (RSV) of aqueous polyacrylate dispersions comprising acrylate copolymers prepared with such monomers).
U.S. Pat. No. 4,659,771 discloses that stable aqueous polyacrylate dispersions can be prepared in the presence of protective colloids when relatively minor amounts of a substantially completely water-soluble monomer possessing conjugated unsaturation is present. In addition, U.S. patent application Ser. No. 06/884,822, now U.S. Pat. No. 4,966,939, and European Patent Application Nos. 252,526 and 331,011 disclose that minor amounts of water-soluble monomers possessing conjugated unsaturation are capable of stabilizing aqueous polyacrylate dispersions by themselves (i.e., in the absence of a protective colloid), either in the presence of or absence of comonomers, including organo-soluble allyl monomers. The RSV, which is indicative of the molecular weight due to carbon chain length (as opposed to weight added due to branching), of acrylate copolymers prepared according to the aforementioned documents is comparable to conventional polyacrylate dispersions.
There is presently a desire for dispersions of polyacrylates having higher molecular weight due to increased carbon chain length and improved moisture resistance, such as that imparted by the aforementioned wet adhesion monomers. It is known that molecular weight of latex polymers may be increased by lowering initiator concentration, lowering reaction temperature, adding multifunctional monomers or performing any combination of these steps. However, each of these steps has undesired side effects, such as slowing the rate of monomer conversion, leading to longer reaction times for the first two methods; and increasing polymer branching, leading to reduced rates of film formation and possible loss of gloss for all three of these methods.
Accordingly, it is an object of this invention to provide acrylate copolymers and aqueous polyacrylate dispersions of improved processability and improved polymer molecular weight (due to longer polymer chain length). In addition, it is an object to provide acrylate copolymers and polyacrylate dispersions of improved film toughness and clarity. Further, it is an object of this invention to prepare acrylate copolymers having moisture resistance such as that imparted by the aforementioned wet adhesion monomers. Other goals and objectives of this invention should be evident to the person of ordinary skill in the art from the above discussion and the following disclosure.